The present invention relates to sputtering target assemblies comprised of a sputter target bonded to a backing plate. More particularly, the present invention relates to methods for quantitatively evaluating sputter target/backing plate bond quality, and to systems for performing nondestructive bond evaluation of sputtering target assemblies.
In the sputter application field, a sputtering target assembly typically includes a sputter target and a backing plate. For instance, a metal target or metal target blank (e.g., tantalum, titanium, aluminum, copper, cobalt, tungsten, etc.) is bonded onto a backing plate. The backing plate can be, for example, a backing plate flange assembly such as copper, aluminum, or alloys thereof. Among the factors that can affect sputtering performance of a given sputtering target assembly is the degree of thermal and electrical contact maintained between the sputter target and the backing plate during the sputtering process. To achieve the desired thermal and electrical contact between the sputter target and the backing plate, the sputtering target assembly members are bonded or attached to each other by conventional means such as soldering, brazing, diffusion bonding, clamping, explosion bonding, friction welding, press fitting, epoxy cementing, and the like. The degree of thermal and electrical contact achieved in the bonding process can depend on the quality of the bond throughout the entire bond interface located between the bonding surfaces of the sputtering target assembly members.
The sputter target and the backing plate are typically made from materials having dissimilar coefficients of thermal expansion. The differential expansion between the target material and the backing plate material that occurs when bonding is accomplished at elevated temperatures such as by soldering, brazing, or diffusion bonding, can generate very high levels of mechanical stress in the metal of the sputtering target assembly members. The mechanical stress can cause deflection of the sputtering target assembly, leading to separation of the sputter target from the backing plate due to bond failure. Bond failure due to poor bonding can occur anytime during handling but is most likely to occur during use, when bond strength or shear strength can be at a minimum due to exposure to relatively high temperatures attained in the sputtering process. The debonding risk is even more possible due to the continuing progression of the industry to use larger and larger sputter targets.
For at least the reasons discussed, i.e., sputtering performance and shear failure, sputtering target assemblies bonded by conventional methods are typically inspected for bond quality prior to use to assure that the bond integrity is satisfactory. Ultrasonic scanning or testing (UT) is typically used in evaluating the bond integrity between the target and the backing plate in a sputtering target assembly. In UT, the target/backing plate assembly is immersed in water and an ultrasonic transducer operating between about 1 to 30 MHz, is used to scan in an x-y raster pattern over the sputtering target assembly surface. Bond integrity throughout the bond interface can be determined by measuring the ultrasonic reflection from the target/backing plate interface. For areas in which no strong reflection is detected, the bond is deemed to be sound. Alternatively, if a strong reflection is detected, the associated regions are deemed to be poorly bonded.
Scanning time for UT of sputtering target assemblies is proportional to the area being scanned. A typical scan rate for a sputtering target assembly is approximately 15 to 30 cm2/min. A typical sputtering target assembly, e.g. for coating 200 mm silicon wafers for semiconductor fabrication, can be about 1,000 cm2. Thus, typical scan times are 30 to 60 minutes. Evaluations of this duration are disadvantageous for numerous reasons. In addition, the accuracy of the results obtained by ultrasonic testing can be related to the manner in which the scanning is conducted. Further, it is advantageous to reduce the data obtained regarding debond location to a readily observable medium such as a graphic image.
Accordingly, a need exists for a method of evaluating the bond integrity of a sputtering target assembly that requires less time to perform than currently used evaluation methods. A need also exists for a system for performing non-destructive examination of a sputtering target assembly that provides a quantitative analysis in which the accuracy of the results are not subject to the inherent limits of mechanical scanning. A further need exists for a method to present assembly debond location information in a representative map.